It applies to both roaming terminals and roaming networks. The IP mobility of a terminal is usually managed in the terminal itself, whereas the mobility of a network is managed by a specific item of network equipment called a mobile router. In the present application, the term “node” indicates either a mobile router or a roaming terminal.
“Home network” means the IP network to which the mobile network or terminal is attached when it is not roaming. “Visited network” means the IP network to which the terminal or network is connected when it is in a mobile situation.
“Home agent” means the entity of the home network with which a roaming node registers its temporary address. It is responsible for forwarding the data streams to roaming nodes.
The mobile node has a reference address configured with the prefix of its home network. If it is a router, this reference address may also be configured based on a prefix allocated to the mobile network (mnp). This address, called the “home address”, is here marked HoA. It may furthermore configure one or more temporary addresses using a prefix of a visited network. Such a temporary address or “care-of address” is here marked CoA.
A node in a mobile situation or roaming usually uses protocols specified by the IETF (“Internet Engineering Task Force”) for communicating by using one of its CoAs, particularly the mobile IP (see “Mobility Support in IPv6”, IETF, RFC 3775, June 2004) and Nemo-BS (see “Network Mobility (NEMO) Basic Support Protocol”, IETF, RFC 3963, January 2005) protocols. Communicating means clearly a two-way communication. This mobile node is then attached to a visited network.
Management of IP mobility of the nodes is based on the temporary allocation of an IP address to the node when the latter is in a visited network. This address is communicated to the home agent which is responsible for forwarding to the temporary address the data streams intended for the roaming node. This means that the home agent must know the “position” (the temporary address) of the node every time the latter changes its network of attachment. The roaming node must therefore inform its home agent of its new temporary address when it perceives a change of network.
The Mobile IP and Nemo-BS protocols are based on an anchoring principle by which the home agent of the mobile node is responsible for relaying the traffic to it when this node is attached to visited networks. The mobile node has a registration associating its temporary address (CoA) with its home address (HoA). In order to keep the association up to date during a change of network, a three-step transfer procedure, or “handover”, has been defined:    1. Detection of the movement: the mobile node ascertains that it has changed network of attachment while roaming;    2. Address configuration: the mobile node configures a temporary address (CoA);    3. Association update: the mobile node informs the home agent of its new location (CoA).
During its roaming, it is possible for a mobile terminal to configure several temporary addresses corresponding to one or more access routers. However, Mobile IP provides for only one association (HoA, CoA) to be active (registered) at a given moment. Therefore, before sending an association update message (step 3), the mobile node has already chosen its CoA. The home agent can only acknowledge and relay the traffic in the event of success.
Selecting a CoA in itself is not governed by a real rule. The choice of one or other of the temporary addresses is left to the mobile node which changes the association only after having verified that the preceding association is no longer usable, usually in the case in which the corresponding access router can no longer be reached.
For mobile networks, mobility management according to Nemo-BS provides no details relating to the selection of a CoA. However, it involves routers for which the choice may have a strong influence on the conditions of use in the mobile network.
The CoA selection procedure is not carried out only at the time of a handover. A mobile node that starts up carries out the three steps described above and may be required to make a choice of CoA.
A mobile node may today know certain characteristics of the attachments that can be used. Various tasks are being conducted for the detection of new access networks, the determination of an identity for each link found and of the prefix or prefixes that it uses, and the detection of the capabilities of the routers (compression, access control, etc.).
The document Park, et al., “Link Characteristics Information for Mobile IP” published by the IETF as an Internet Draft in June 2005 (draft-daniel-mip-link-characteristic-02.txt) describes the uploading by the mobile node of certain information relating to the attachments. It defines a mobility option for uploading to a correspondent or the home agent the attachment characteristics of a given type (IEEE 802.11a, IEEE 802.11b, cellular, etc.). This allows the correspondent or the home agent to adapt its behavior to suit the attachment characteristics that have been indicated to it so that, for example, it does not continue to send high speed data to a mobile node that may have switched to a low speed link, which would cause traffic congestion. This process is independent of the selection of the CoAs.
As defined today, IP mobility management provides only an incomplete solution for reducing the multiple address configuration alternatives. In particular, the registration of associations has limitations because a new CoA is registered only when the old one can no longer be used, that is to say when the corresponding access router can no longer be reached.
The possibility offered to the mobile nodes to configure several temporary addresses originating from several attachments (various access routers belonging if necessary to visited fixed or mobile networks) means that the mobile nodes may have a choice of access types with potentially very nonuniform intrinsic characteristics. These accesses may differ according to several criteria: nominal quality of service, load conditions, functionalities supported, provider, etc. For reasons of service, reliability or strategy, it is advisable for the mobile node to prefer one or other of these accesses according to specific criteria. A policy of choice must therefore be put in place.